U.S. Pat. Nos. 4,570,735 (Duffy) and 4,570,736 (Waldorf) disclose variable assist power steering systems for automotive vehicles. Each of these systems incorporates a hydraulically operated power steering valve assembly which receives hydraulic fluid from a source. The valve assembly provides a greater steering assist when the vehicle is parked or travelling at low speed than when it is travelling at high speed. Speed sensitive valving or switching is provided to control the delivery of hydraulic fluid from the source to the valve assembly to thereby achieve the desired speed related variable assist feature. The valving and switching techniques of the foregoing systems are complex, and therefore costly and subject to reliability problems. Further, it is known that stepping motor linear actuators have been employed to control the flow of hydraulic fluid to variable assist power steering valve assemblies because of the problems associated with the control systems of the foregoing patents. Such a system is described in an SAE paper entitled "1988 Lincoln Continental Variable Assist Power Steering System" (J. J. Duffy). However, stepper motor systems are rather large and costly and they require complex control electronics. Further, when any stepper motor system fails, it will remain in the operating state it was in at the time of failure, which is not an inherently failsafe failure mode.